In the guidance of movable measurement carriages of coordinate-measuring machines, air bearings are frequently used as the means of supporting the involved machine part on its guide. The loading of involved air bearings is a function of the gravitational weight of the guided part of the machine.
Relatively large loading forces are required in order to operate air bearings at their optimal working point, namely, the point at which they have the greatest possible stiffness. Necessarily, therefore, the guided part must be of a relatively great weight. But weight, on the other hand, imposes limitations on the obtainable accelerations or speeds with which the part can be moved.
The bearings of guided machine parts are frequently clamped via abutments which are under spring load. However, this technique is only economically feasible if the guided part is movable linearly in a single direction. On the other hand, if the involved guide is a so-called planar guide on which the machine part is movable simultaneously in two degrees of freedom, then it is practically impossible to provide the spring-loading which is necessary for the abutment.
So-called self-priming air bearings are known from U.S. Pat. No. 4,558,909. These air bearings are provided on their periphery with a suction channel which can be connected to a vacuum source, to draw the bearing against the involved guide surface. However, the preloading forces that can be produced with this known bearing are relatively slight, being less than optimal since the suction channel has an area that is less than that of the air bearing, and the pressure difference to produce the suction force is at most one atmosphere. Such slight forces are generally not sufficient to operate the bearing in the operating range which is optimal for the greatest possible stiffness.
Federal Republic of Germany OS 2,829,715 describes a coordinate-measurement machine having machine parts which are guided via air bearings, and which are also magnetically loaded against their guide surfaces. Such construction requires that the involved guide contain ferro-magnetic material. This is not advantageous, for various reasons.